In recent years, various kinds of information equipment have been widely used which require disks an increased storage capacity, boosting to develop devices with a higher storage density, a compact sizing and low profile designing. Moreover, along with advances of downsizing and low profiling of the devices, a wire diameter for the coil wound on iron core of the stator of spindle motor has decreased and various winding methods for the wire has been developed.
Next, an iron core of a stator for conventional spindle motor used in hard disk drives, optical disk drives or the like is described schematically with reference to drawings. FIG. 9 shows a schematic cross sectional view taken along a plane perpendicular to a rotation center of a portion of stator and rotor magnet of a conventional spindle motor.
In FIG. 9, an iron core comprises: a plurality of magnetic poles 94 having pole-tops 91 at both sides peripherally on the top facing rotation center 1 inwardly and straight portion 93 to wind a coil; and pole base 95 to join respective magnetic poles 94 outwardly. A plurality of iron layers made of for instance silicon steel plates or the like are laminated to form iron core 96; core 96 and coil 92 forms stator 97. The outward periphery of rotor magnet 98 including a plurality of magnetized sections faces the inward periphery of magnetic poles 94 of stator 97 across an air gap. Upon energizing, as well known, coil 92 generates magnetic fluxes allowing rotor magnet 98 including a plurality of magnetized sections to rotate. In such configuration having stator 97 and rotor magnet 98, the air gap between the inward periphery of magnetic pole 94 and rotor magnet 98 varies abruptly in the vicinity of clearance between pole-tops 91 of neighboring magnetic poles 94, or slots 99, causing the magnetic flux density to vary abruptly. Consequently, attractive forces between magnetic poles 94 and rotor magnet 98 vary abruptly causing cogging torques and ripple occurs in motor rotations. To constrain the abrupt variations in magnetic flux density between magnetic pole 94 and rotor magnet 98 in the vicinity of slots 99, pole-top 91 is formed such that a distance to surface 91a from rotation center 1 becomes smaller as it extends peripherally.
Additionally, examples of iron core design to constrain the generation of cogging are:
an inward periphery for a magnetic pole of the iron core has a larger radius of curvature than a distance between an intersection of the inward periphery with the centerline of the magnetic pole through the rotation center,
or an inward periphery for a magnetic pole of the core has a plane perpendicular to the centerline of the magnetic pole (for instance, see Japanese Patent Unexamined Publications No. H8-111968 and H11-987920). Such core configurations can widen the air gap between the magnetic core and rotor magnet gradually from the center of inward periphery toward both ends of the pole-top. This results in gradual variations in attractive forces between magnetic poles 94 and rotor magnet 98 enabling motor to reduce cogging torque at rotation.
However, the conventional iron core configuration for the spindle motor has the problems of decreases in motor efficiency, as the air gap between the magnetic core and rotor magnet widens gradually from the center of inward periphery toward both ends of pole-top and that a distance to the surface of projection facing the pole-base from the rotation center becomes smaller as it extends peripherally.
Additionally, along with the progress in downsized and low profiled devices, the iron core of spindle motor requires a very thin wires to wind a coil and the regular winding technology using thin wires has become of great importance. However, in the iron core configuration of the stators shown in FIG. 9, surface 91a of pole-top 91, facing pole-base 95, intersects with straight portion 93 obtusely, causing difficulties to wind coils 92 using very thin wires on straight portion 93 of magnetic pole 94 regularly. Even if wound regularly, the regular windings of coil 92 is broken or likely to be broken due to a slight slack of the winding in the processes of winding, assembling after winding or at motor operation, causing difficulties to dispose the coils in a predetermined position properly. The problem is that in an extreme case coil 92 touches rotor magnet 98 owing to the broken coil windings to cause failures such as damaging the insulation layers of coil or the like resulting in a poor reliability of the motor operation.